Dual frequency cavity backed slot antenna



April 1967 B. F. GREGORY 3,312,976

DUAL FREQUENCY CAVITY BACKED SLOT ANTENNA Filed July 19, 1965 RADIATIONINVENTOR.

54 346244 1112217: [7' Gregory 3 BY 14 Blair 5, ac/(Zes 2 -*l 12Nam/mas,

United States Patent 3,312,976 DUAL FREQUENCY CAVITY BACKED SLOT ANTENNABenjamin F. Gregory, Tampa, Fla, assignor to Trak Microwave Corporation,Tampa, Fla. Filed July 19, 1965, Ser. No. 472,932 9 Claims. (Cl.343-767) This invention relates to microwave radiators, and moreparticularly to a microwave antenna structure for projecting two or moresignals of preselected different frequencies in broad beams from thesame source of origin.

The problem to which the invention is directed is to provide a small,lightweight, flashlight type of microwave radiator capable of being handheld and of projecting beams of invisible microwave energy in much thesame manner as an ordinary flashlight projects visible light.Furthermore, the microwave flashlight of the invention is capable ofprojecting two or more beams of different wave lengths and frequencyalong a common central axis, either independently of each other orsimultaneously, or alternately, depending upon various applications forwhich the device may be employed. The device of the invention is usefulfor providing a portable source of microwave energy for field testingvarious types of microwave apparatus.

Accordingly, it is an object of the invention to provide a portablemicrowave antenna suitable for projecting a unidirectional broad beam ofradio frequency energy from a small, hand held, source.

A more particular object is to provide a device of the above characterin which the back wave of radiant energy is terminated withoutabsorption or loss of energy from the source.

A further object is to provide a device of the above character in whichenergy of more than one frequency is radiated with maximum efficiency ina beam along a common axis.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts which will beexemplified in the constructions hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings in which:

FIGURE 1 is a front view of the antenna of the invention with theprotective cover removed;

FIGURE 2 is a front view of the antenna structure with both protectivecover and cavity cover removed, showing a plan view of the internalconstruction;

FIGURE 3 is a cross-sectional view of the device taken along the line3-3 of FIGURE 2, showing internal construction of the antenna, with theprotective insulating cover in place;

FIG'URE 3A is a diagrammatic representation of the voltage distributionwithin the cavity 15 of FIG. 3, showing the direction and mode ofelectromagnetic radiation from the slot 11 thereof.

Referring now in greater detail to the drawings, the antenna structureof the invention comprises a circular front plate of electricallyconductive metal, having pierced therethrough two narrow slots 11 and12, as shown in FIGURES l and 3. The lengths of the slots 11 and 12, asshown in FIGURE 1, are each made to be approximately one-half of thewave length of energy to be radiated theret hrough.

The front plate 10 is mounted on and supported by a cylindrical metalblock 14, having cut into the front surface thereof two substantiallysemi-cylindrical cavities 15 and 16, as shown in FIGURES 2 and 3. Theslot 11 overlies and opens into the cavity 15, while the slot 12overlies and opens into the cavity 16. The cavities 15 and 16 areseparated from each other by conductive wall portion 17 of block 14. Thethickness of wall portion 17 in the preferred embodiment is equal to thespace between slots 11 and 12, as shown in FIGURE 3, although thiscondition is not essential to the invention.

A coaxial RF conductor line 18, having an inner conductor 19, is passedthrough a hole 20 in the block 14 and is securely held by a recesesd setscrew 21 in the sidewall of block 14. The inner conductor 19 extendsthrough the cavity 15 into the front plate 10 where .it is electricallyconnected by a solder well 22. The position of connection 22 is locatedon the radius of semi-cylindrical cavity 15 normal to the wall 17, at apoint where the impedance reflected back into coaxial line 18-19 isfifty ohms. The radius R of cavity 15 is approximately equal to onequarter of the wave length of radio frequency energy fed to the antennaover coaxial line 13-19 for radiation through slot 11. This creates acondition of zero impedance, or short circuit, along the curved wall ofcavity 15 at point 24, and maximum impedance at point 23 along the planesurface 25 at the midpoint of straight wall 17. It will be understoodthat in order to obtain a fifty ohm connecting point 22 at a convenientposition approximately inthe center of cavity 15, the impedance at point23 must be substantially greater than fifty ohms. This is accomplishedby providing suflicient volume in the cavity 15; the greater the volume,the higher the impedance will be at point 23. The impedance measuredbetween the points 20A and 20B, as shown in FIGURE 3, was empiricallydetermined to increase as the volume of cavity 15 is increased. Themaximum impedance of the slotted antenna structure occurs between thesurfaces 11 and 11A as shown in FIGURES 1 and 3. While the depth of thecavity 15 is not critical, I have found that a depth D, as shown inFIGURE 3, approximately equal to one sixteenth of the wave length ofenergy to be radiated, produces a fifty ohm connecting point 22 at thedesired position for maximum efiiciency of electromagnetic radiation andfor minimum reflection of back wave along the coaxial line 18-19.

To enable fine adjustment of cavity 15, for accurate tuning of theantenna to the exact frequency at which microwave energy is to beradiated, I provide a tuning plug 26 threaded through a tapped hole 27in the rear wall of cavity block 14. A similar, but smaller diameter,tuning screw 28 is threaded through block 14 into the smaller cavity 16,as shown in FIGURE 3 of the drawing.

As cavity 16 is designed to radiate microwaves through slot 12 of a muchhigher frequency than those from cavity 15 through slot 11, cavity 16 isproportionately smaller than cavity 15. The radius r of cavity 16 isapproximately one-quarter wave length at its frequency, the slot 12 isapproximately one-half wave length long, and the depth d of cavity 16 isapproximately one-sixteenth wave length. Microwave energy of theappropriate higher frequency is fed to the antenna cavity 16 through asecond coaxial conductor 29-30 which is secured in cavity block 14 by aset screw 31. The inner coaxial conductor 30 is passed through cavityv16 and soldered to front plate 10 at point 32, to reflect a fifty ohmimpedance in the same manner as described above in reference to theconnection of coaxial line 18-19 at point 22.

The slots 11 and 12, opening into cavities 15 and 16 respectively, areprotected from the intrusion of moisture, or other foreign matter whichcould affect the tuning of the antenna, by a dielectric cover plate 34which is fiber glass. The dielectric cover plate 34 is adhered to theface of conductive plate by any suitable means, such as resin cement.

It will be appreciated that inasmuch as the radiation resistance of freespace is approximately three hundred and seventy seven ohms, and theimpedance looking into the slots 11 and 12 is also approximately 377ohms at the frequencies to which the respective cavities 15 and'16 aretuned, by the herein disclosed antenna design I have provided means foreffectively coupling microwave energy of two discrete frequencies fromconventional fifty ohm lines into space, for efficient radiation inunidirectional beams along a commonaxis, with minimum back wavereflection.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in the above construction withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted 'as illustrative and not in a limitingsense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention which,as a matter of language, might be said to fall therebetween.

Having described my invention, what I claim as new and desire to secureby Letters Patent is:

1. A unidirectional microwave antenna radiator comprising incombination,

(A) a semi-cylindrical cavity substantially enclosed by electricallyconductive material,

(B) a slot opening in to said cavity along the diameter of said cavityand adjacent a high impedance portion of said cavity, and

(C) means for introducing radio frequency energy into said cavity at aselected lower impedance point within said cavity, whereby said radiofrequency energy resonates in said cavity and is emitted through saidhalf wave slot in a directional beam.

2. A unidirectional microwave antenna according to claim 1 wherein,

(B) said slot is substantially one half wave length of the energy to beradiated, and

(C) said introducing means is a low impedance coaxial line.

3. The combination of claim 1 including a manually adjustable tuningscrew inserted into said cavity for tuning the resonant frequencythereof.

4. The combination of claim 2 including a manually adjustable tuningscrew inserted into said cavity for tuning the output frequency of saidslot.

5. A multiple radiator microwave antenna comprising, in combination,

(A) a pair of substantially semi-cylindrical cavities formed within anelectrically conductive member,

(1) each of said cavities having at least one plane wall lying in acommon plane with the other,

(B) a substantially straight conductive wall separating said cavitiesalong their respective diameters,

(C) a pair of narrow slots in said common plane wall adjacent toopposite sides of said separating wall, and passing through saidcommonplane wall,

(1) each of said slots opening into a separate one of said cavities, and(D) means for introducing radio frequency electromagnetic energy intoeach of said cavities whereby said energy resonates in said cavities andemanates justable tuning means in each of said cavities.

8. The combination of claim 5 in which,

(D) said means for introducing radio frequency energy into each cavitycomprises separate coaxial lines separately connected with saidcavities.

9. A multiple frequency microwave antenna comprising in combination,

(A) a pair of substantially semi-cylindrical cavities formed within anelectrically conductive member,

(1) each of said cavities formed with a different radius (2) each ofsaid cavities having a wall thereof lying in a common plane.

(B) a substantially straight wall of conductive material normal to saidcommon plane and separating said cavities substantially along theirrespective diameters,

(C) a pair of parallel narrow slots in said common pl'alrlie adjacent toopposite sides of said separating wa (1) each said slot being of alength substantially equal to one half wave length at the frequency tobe radiated, and

(2) extending along the diameter of said cavity,

(D) an adjustable tuning screw protruding into each of said cavities fortuning the resonant frequency of each cavity, and

(E) separate coaxial lines connected with each of said cavities,

(1) each of said coaxial lines being connected to its respective cavityat a point of low impedance,

(2) whereby electromagnetic energy of preselected radio frequencies maybe introduced into each of said cavities and caused to radiate throughsaid slots in beams perpendicular to the plane of said common planecavity walls.

References Cited by the Examiner UNITED STATES PATENTS 2,741,763 4/1956Ashwell et a1 343767 2,791,769 5/1957 Lindenblad 343770 2,885,676 5/1959Ba'lwin 343-767 3,056,130 9/1962 Charman 343-767 3,262,119 7/1966 Sisson343-768 X ELI LIEBERMAN, Primary Examiner.

R. F. HUNT, Assistant Examiner.

1. A UNIDIRECTIONAL MICROWAVE ANTENNA RADIATOR COMPRISING INCOMBINATION, (A) A SEMI-CYLINDRICAL CAVITY SUBSTANTIALLY ENCLOSED BYELECTRICALLY CONDUCTIVE MATERIAL, (B) A SLOT OPENING IN TO SAID CAVITYALONG THE DIAMETER OF SAID CAVITY AND ADJACENT A HIGH IMPEDANCE PORTIONOF SAID CAVITY, AND (C) MEANS FOR INTRODUCING RADIO FREQUENCY ENERGYINTO SAID CAVITY AT A SELECTED LOWER IMPEDANCE POINT WITHIN SAID CAVITY,WHEREBY SAID RADIO FREQUENCY ENERGY RESONATES IN SAID CAVITY AND ISEMITTED THROUGH SAID HALF WAVE SLOT IN A DIRECTIONAL BEAM.